| Laser powder cladding is an additive process where a powder is melted and solidated by the use of laser beam energy in order to coat a workpiece surface to obtain the required properties.Due to the technological characteristics of rapid melting and rapid solidification,a great amount of residual stress is formed inside the components,which will lead to failure problems such as deformation of cladding components and cladding layer cracking.In this paper,YAG laser is used to cladding Ni60 powder on thin substrate.The correlation between the residual stress distribution and the deformation of cladding parts are established through the experimental study,which is benefit to reveal the mechanism of residual stress in laser cladding.In this paper,samples were prepared by changing the process parameters including laser power,scanning speed and number of scanning number.The bending deformation of the samples under different parameters was measured,and the residual stress of the cladding layer and the bottom of the substrate was tested by X ray diffractometer(XRD).Thermocouples were used to monitor the temperature change of samples in the cladding process,and the variation of temperature gradient and heat accumulation was analyzed.The depth of the melten pool and the microstructure of the cladding layer were observed by laser spotlight microscope and scanning electron microscope(SEM).Finally,the residual stress was reduced by heat treatment,remelting and scanning the bottom of the substrate.The best process under the current experimental conditions was obtained.Experimental results show that:(1)As the laser power and the number of scanning number increased,the bending angle of the substrate and the residual stress of the cladding layer gradually increased.As the scanning speed increased,the bending angle of the substrate and the residual stress of the cladding layer decreased at first and then increased.When the laser power was 1300 W,substrate had the maximum bending angle:11.3°,and the residual stress of the cladding layer was maximum:?_x=576.3 MPa(Parallel to the scanning direction is defined as the X direction)??_y=-448.2 MPa(Perpendicular to the scanning direction is defined as the Y direction).When the number of scanning number was 11,substrate had the largest bending angle:9.12°,and residual stress of the cladding layer was maximum:?_x=324.2 MPa??_y=-311.0 MPa.When the scanning speed was 70 mm/min,substrate had the minimum bending angle:9.12°,and the residual stress of the cladding layer was minimum:?_x=324.2 MPa??_y=-311.0 MPa.(2)The temperature gradient in the X direction was smaller than that in the Y direction through analysis,which was one of the reasons for the deformation in the Y direction.In the cladding process,there was heat accumulation in the samples.As the number of cladding track increased,the heat accumulation factor gradually increased,but the increasing amplitude gradually decreased and tended to balance.The greater the heat accumulation,the greater the depth of fusion.The distribution range of planar crystals becomed smaller and smaller,the distribution range of equiaxed crystals becomed larger and larger,and the distribution range of dendrites becomed smal er and smaller in the same sample.(3)The residual stress of the sample was eliminated by heat treatment,re-melting and scanning the bottom of the substrate.Under the current experimental conditions,laser scanning the bottom of substrate had the best residual stress elimination effect.The best laser power of scanning the bottom of the substrate was 450 W,bending angle of the sample was6.00°smaller than that of untreated sasmple,residual stress relieved by 81.82%(X direction),79.15%(Y direction).Compared to the optimal heat treatment parameters,decreased bending angle was more than 1.64°,stress eliminating was more than 10.45%(X direction),9.41%(Y direction);Compared with the optimum parameters of the remelting,decreased bending angle was more than 0.98°,stress was more than 35.20%(X direction),33.95%(Y direction). |
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